Third generation (“3G”) mobile communication systems, such as cellular telephone systems, predominantly employ Code Division Multiple Access (“CDMA”) methods. Wideband CDMA (“WCDMA”) is a widely deployed version of CDMA employed in Universal Mobile Telecommunications System (“UMTS”) networks.
CDMA is a variety of direct sequence spread spectrum communications. A CDMA signal occupies a bandwidth much greater than is necessary to send the information carried by the signal, and as a result possesses a measure of interference immunity and multi-user access capability. CDMA uses unique pseudo-noise (“PN”) codes, also known as spreading codes, to increase the bandwidth, or spread the baseband data before transmission. The PN code sequences are produced at a much higher rate than the baseband data and different codes are applied to each user's data within a cell. The bits of a spreading code are known as “chips” to distinguish them from data bits. The ratio of the chipping rate (i.e., the inverse of the chip period) to the baseband data rate is known as the spreading factor. The PN codes assigned to different users within a cell are selected to minimize cross-correlation between the codes. This allows the signal receiver to extract a user's data from the aggregate received signal by correlating the received signal against the spreading code assigned to the user. The process of correlating received signal with a known spreading code is called despreading.
In practical applications of a mobile communications system, transmissions between devices, for example between a mobile station, also known as a mobile terminal, a user equipment (“UE”), etc., and a base station, also known as a base transceiver station, a fixed terminal, a Node B, etc., encounter a variety of reflective surfaces (e.g., buildings, vehicles, etc.). These surfaces reflect a transmitted signal causing multiple copies of the signal to arrive at the receiver at different times and with varying amplitudes. Rather than attempting to suppress the multi-path signals, CDMA systems use the multi-path signals to improve receiver performance using rake demodulation.
Rake demodulation comprises separately despreading the different resolvable multi-paths, or “fingers,” of a CDMA radio link, and constructively combining them to produce a signal having improved signal-to-noise ratio. A rake receiver performs finger despreading by correlating a given antenna sample stream with the channel's PN code shifted by the delays identified for each finger.
Correlation techniques are extensively employed in rake receivers. In addition to finger despreading, a variety of other tasks including early-ontime-late (“EOL”) finger tracking, path searching, and preamble detection rely on correlation. A rake receiver may be provided with hardware accelerators to facilitate the correlation tasks. For example, one or more correlation coprocessors may perform despreading and finger tracking, while one or more additional correlation coprocessors may execute path searching. A set of specific correlation coprocessors may be dimensioned for the nominal distribution of the various correlation tasks encountered in a given cellular environment. While this strategy provides optimal correlation performance for the nominal task distribution, it results in degraded performance for extreme distributions.